System and method for unscheduled wireless communication with a medical device

ABSTRACT

Unscheduled wireless communication with a medical device is achieved by operating a receiver of the medical device in a series of modes. Each mode provides an increasingly selective evaluation of received RF energy. The receiver, when operating in a first mode, is capable of detecting the presence of RF energy transmitted from a communicating device. The receiver, when operating in a second mode, consumes more energy than the first mode and analyzes the RF energy to determine whether it contains the appropriate type of modulation. When operating in a third mode, the receiver consumes more energy than the second mode, and operates the full receiver to begin communication with the communicating device. The receiver opens a communication session after the RF energy has passed the evaluation by the series of modes to receive an unscheduled communication.

CROSS-REFERENCE TO RELATED APPLICATION(S)

Reference is made to the following applications, filed concurrentlyherewith: U.S. patent application Ser. No. 11/224,591, now U.S. Pat. No.7,890,191 issued Feb. 15, 2011, entitled “SYSTEM AND METHOD FORUNSCHEDULED WIRELESS COMMUNICATION WITH A MEDICAL DEVICE,” by Quentin S.Denzene and George C. Rosar: U.S. patent application Ser. No.11/224,595, still pending, entitled “COMMUNICATION SYSTEM AND METHODWITH PREAMBLE ENCODING FOR AN IMPLANTABLE MEDICAL DEVICE,” by Gregory J.Haubrich, Javaid Masoud, George C. Rosar, Glenn Spital, and Quentin S.Denzene; and U.S. patent application Ser. No. 11/22,594, still pending,entitled “IMPLANTABLE MEDICAL DEVICE COMMUNICATION SYSTEM WITH MACRO ANDMICRO SAMPLING INTERVALS,” by Glen Spital, incorporated herein byreference in their entireties.

FIELD OF THE INVENTION

The present invention relates generally to implantable medical devices,and more particularly, to wireless communication with implantablemedical devices.

BACKGROUND OF THE INVENTION

Current implantable medical devices (IMDs) provide countless therapiesand monitor a wide variety of physiological events. With the increaseduses of IMDs has also come the need for improved methods ofcommunicating with and between IMDs.

Conventionally, communication with IMDs has been with magnetic fieldcommunication systems. Such systems, however, are generally only capableof communicating over very short distances, on the order of a fewinches. As a result, a magnetic head of a programmer (or other externaldevice) needs to be placed near to the IMD for communication to occur.More recently, radio frequency (RF) based communication systems havebeen developed for use with IMDs. RF communication provides a number ofbenefits over magnetic field communication systems, including muchgreater communication distances. However, conventional RF communicationsystems also consume more battery power than magnetic fieldcommunication systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating bi-directional RFcommunication between an implantable medical device (IMD) and aprogrammer.

FIG. 2 is a block diagram illustrating the RF communication systemcomponents of the IMD and programmer.

FIG. 3 is one embodiment of an RF transmission, transmitted by theprogrammer to initiate an unscheduled communication with the IMD.

FIG. 4 is a block diagram of an antenna and a receiver of the IMD.

FIG. 5 is a spectral diagram illustrating three sequential receivermodes.

FIG. 6 is a flow diagram illustrating the multi-tiered method ofoperating receiver 38.

DETAILED DESCRIPTION

The present invention includes a system and method for unscheduled,automatic wireless communication with a medical device. A medical deviceincludes a receiver capable of being operated in a plurality ofsequential modes. When operating in each consecutive mode, the receiverprovides an increasingly selective evaluation of the RF energy. Afterpassing the evaluation, the receiver receives the unscheduledcommunication.

FIG. 1 is a schematic diagram illustrating bi-directional RFcommunication between IMD 12, which includes lead 14 and antenna 16. Inone embodiment, IMD 12 is an implantable cardioverter defibrillator(ICD), but the present invention is equally applicable to many types ofmedical devices, including both implantable medical devices and externalmedical devices (XMD). IMD 12 is capable of providing therapies and/orsensing physiological events of the heart of patient P via lead 14.Antenna 16 is used to communicate with programmer 18 and may be anydevice capable of sending or receiving electromagnetic waves, includinga surface mounted antenna, inductor, or half-wave strip.

Programmer 18 is an external programming unit capable of bi-directionalcommunication with IMD 12 via antenna 20. In alternate embodiments,programmer 18 may be replaced by any device capable of communicatingwith IMD 12. Antenna 20 may be any type of RF antenna capable ofcommunicating in the desired RF frequencies with IMD 12, and may belocated inside or outside of a housing of programmer 18.

FIG. 2 is a block diagram illustrating some of the functional componentsof IMD 12 and programmer 18 that make up RF communication system 26.Programmer 18 includes antenna 20, programmer circuitry 27, andtransceiver 28. Antenna 20 is coupled to transceiver 28 of programmer18. Programmer circuitry 27 includes a microcomputer and software tocontrol the operation of programmer 18. Transceiver 28, coupled toantenna 20, enables programmer circuitry 27 to transmit and receivecommunications with IMD 12. Transceiver 28 of programmer 18 includestransmitter 32 and receiver 34.

IMD 12 includes antenna 16, IMD circuitry 29, and transceiver 30. IMDcircuitry 29 includes a microprocessor, therapy delivery circuitry fordelivering a therapy through lead 14, and sensors for detectingelectrical signals on lead 14. Transceiver 30, coupled to antenna 16,enables IMD circuitry 29 to transmit and receive communications withprogrammer 18. Transceiver 30 includes transmitter 36 and receiver 38.

Because IMD 12 has a finite battery capacity, an important considerationin the design of RF communication system 26 is the energy efficiency ofIMD 12. A substantial factor in the energy efficiency of IMD 12 is thetime transceiver 30 spends either transmitting or receiving. Thus, anyimprovement in energy efficiency of transceiver 30 will lead toincreased battery life of IMD 12. Energy efficiency is less of an issuein the design of programmer 18, because programmer 18 is generallyconnected to an external power source such as a 120V AC. Therefore,methods of operating transceivers 28 and 30 which reduce the energyconsumption of transceiver 30, even in exchange for additional energyconsumption of transceiver 28, are beneficial.

The ability to handle unscheduled communications between programmer 18and IMD 12 is a desired feature of IMD 12. For example, a caregiver maywant to use programmer 18 during an office visit to retrieve informationfrom IMD 12. However, the capability to handle unscheduled communicationhas conventionally either required significant energy consumption by IMD12 or user intervention.

The present invention provides a more energy efficient, automatic methodof communicating with IMD 12. In particular, the IMD includes a receivercapable of operating in a plurality of modes, each having differentlevels of energy consumption. However, before describing the receiver indetail, the RF transmission by programmer 18 will be discussed.

FIG. 3 (not drawn to scale) is one embodiment of an RF transmission 40that is transmitted by programmer 18 to initiate an unscheduledcommunication with IMD 12. Transmission 40 includes a repeating patternhaving burst period 42, discover period 44, and Listen Before Talk (LBT)period 46.

In one embodiment, burst period 42 is a repetitive on-off-keyed (OOK)sequence equivalent to a series of alternating 0 and 1 bits where eachbit transmission takes fifty microseconds, resulting in a 10 kHzpattern. In other embodiments, burst period 42 can be any repetitivepattern that can be detected by receiver 38 of IMD 12 to verify that areceived transmission was transmitted by programmer 18. Burst period 42generally lasts for a fixed period of time, such as 3 seconds.

Discover period 44 follows burst period 42 and is a period in whichvarious transmissions by programmer 18 and IMD 12 take place accordingto a transmission protocol. As part of this protocol, transmissions byprogrammer 18 are followed by periods of listening, during which periodsIMD 12 may transmit, if desired. In one embodiment, discover period 44includes a period during which programmer 18 transmits for 20.1milliseconds, followed by a period of listening for 15.0 milliseconds.Discover period 44 lasts for a fixed period of time, such as for 2seconds. Following discover period 44 is LBT period 46, described below,which is a 0.7 second period during which no communication takes place.

Typically, IMDs communicate over the Medical Instrument Communication(MICs) band with frequencies ranging from about 402 to 405 MHz. Thisband-range is generally divided into ten channels spaced evenly withinthe band. Prior to transmission over the MICs band, the FCC currentlyrequires that an external device must scan each of the ten channels andidentify for broadcast the channel with the least amount of noise(referred to as the “least interference channel”) or any channel belowan FCC-specified RF level. This process must then be repeated at leastevery five seconds until a communication session is opened with adevice. The period of time in which the external device scans thechannels of the MICs band defines LBT period 46. Although the presentinvention is described with reference to communication over the MICsband, the invention is applicable to other communication bands.Alternatively, RF transmission 40 includes only a single LBT period 46,resulting in alternating burst and discover periods 42 and 44 for theremainder of RF transmission 40.

FIG. 4 is a block diagram receiver 38 of IMD 12 along with antenna 16for reception of RF transmission 40 from programmer 18. Receiver 38includes RF module 50 and telemetry digital hardware 52, which in oneembodiment are separate integrated circuits. RF module 50 includes RFcircuitry 54, Received Signal Strength Indicator (RSSI) 56, and serialbus/control 58.

RF circuitry 54 comprises one or more channels of receiver componentssuch as voltage controlled oscillators (VCO), amplifiers, mixers,oscillators, and detectors that enable RF circuitry 54 to receive an RFsignal from antenna 16, filter the signal, and remove the RF carrierfrequency. RSSI 56 receives an output from RF circuitry 54 and providesan analog or digital output indicative of the strength of an RF signalreceived by antenna 16. Serial bus/control 58 provides control andserial communication capabilities to receiver 38. Serial bus/control 58includes RAM buffers, control and status registers, and interrupt lines,as well as a synchronous bus and transmit and receive serial lines.

Telemetry digital hardware 52 includes converter/correlator 60, filter62, microprocessor 64, and serial output 66. Converter/correlator 60converts the output received from RSSI 56 to DC data (if an analogoutput) and determines how well correlated the received data is to anappropriate pattern. The output of converter/correlator 60 is filteredby filter 62 before it is sent to microprocessor 64. Microprocessor 64includes a processor, state machines, logic, and random access memory(RAM) and functions as both the controller of receiver 38 and theinterface between receiver 38 and other components of IMD 12.Microprocessor 64 receives the filtered output from filter 62. Serialoutput 66 operates to coordinate communications between RF module 50 andtelemetry digital hardware 52. The interaction of these various elementsof receiver 38 will be described further with reference to FIGS. 5 and6.

FIG. 5 is a spectral diagram of the MICs band from 402 MHz to 405 MHz,and illustrates the three receiver modes (Modes 1-3) of receiver 38.Within the MICs band are ten communication channels (P1-P10). The centerfrequency of each channel is spaced about 300 kHz from the centerfrequency of the next channel. For example, channel P1 is centered at402.15 MHz, channel P2 is centered at 402.45 MHz, and channel P10 iscentered at 404.85 MHz. As described above, FCC regulations require thatprogrammer 18 broadcast on the least interference channel or any channelbelow the FCC minimum RF level. As a result, IMD 12 does not know priorto transmission which channel will be chosen by programmer 18. In thefigure, communication channel P6, depicted by an open box, contains RFtransmission 40, and channels P1-P5 and P7-P10, depicted bycross-hatched boxed, do not contain RF transmission 40.

The operation of receiver 38 depends on which of the three receivermodes it is currently operating in. When operating in Mode 1, receiver38 operates in a wide bandwidth receiver mode that enables it to receivesignals from two consecutive channels at once plus about 10 kHz. Thebandwidth of the receiver is denoted by bandwidth 67. As shown in FIG.5, the receiver operating in Mode 1 simultaneously samples twoconsecutive channels (such as P1 and P2 with bandwidth 67(a)). Duringeach sample, receiver 38 checks for the presence of RF energy withinthose channels. Receiver 38 then proceeds through the five pairs ofchannels (67(b) through 67(e)) until each pair has been sampled. Becausereceiver 38 only performs a simple analysis of energy present, andbecause of the small sample time, receiver 38 operating in Mode 1requires very low energy consumption. In this example, receiver 38 woulddetect the transmitted energy during sampling of channels P5 and P6, dueto the presence of RF transmission 40 on channel P6.

Receiver 38 adjusts to the appropriate receiver frequencies by adjustinga VCO (not shown) of RF circuitry 54. In one embodiment, receiver 38adjusts the VCO between five different voltages, each voltageapproximately that needed to adjust receiver 38 to center on one of thefive channel pairs. In another embodiment, the VCO is swept graduallythrough a voltage range encompassing the five voltages, such thatreceiver 38 passed through each of the five channel pairs. In anotherembodiment, receiver 38 adjusts the VCO between five different voltages,but then attempts to lock on to a received signal frequency, if any.

When RF energy is detected by receiver 38 in Mode 1, receiver 38 nextadjusts itself to operate in Mode 2. In this mode, receiver 38 operatesin a medium bandwidth receiver mode, represented by bandwidth 68, sothat it receives RF energy of only those frequencies within a singlechannel plus about 30 kHz. In this example, where RF energy is initiallydetected on either channel P5 or P6, receiver 38 need only sample thosechannels. If energy is initially detected in more than one pair ofchannels, then additional samples may also be taken. In Mode 2, receiver38 performs a more rigorous evaluation of any received energy. Receiver38 samples both channels and determines in which channel energy ispresent, and also whether its modulation conforms to the expected typeof modulation of RF transmission 40. This will be described in moredetail with reference to FIG. 6. In the example of FIG. 5, energy isdetected in channel P6, and is found to have the appropriate modulation.

After detecting that energy present on channel P6 contains theappropriate modulation, receiver 38 can be fairly confident that theenergy is RF transmission 40 by programmer 18. As a result, receiver 38enters Mode 3, a full or high power receiver mode in which receiver 38is adjusted to receive energy from only a single channel, represented bybandwidth 69. Receiver 38 receives the signal and determines the contentof the transmitted message. If appropriate, a communication session isthen opened with programmer 18.

In an alternate embodiment, receiver 38 maintains statistical recordsindicating which channels are most often selected by programmer 18 fortransmission and/or which channels most often result in a falsedetection. These records can then be used by receiver 38 to adjust theorder of the channel scan in Mode 1 to improve the response time ofreceiver 38. In this embodiment, receiver 38 begins each Mode 1 channelscan with the pair of channels that are most often selected byprogrammer 18 for transmission. It proceeds through the channels inorder from the channels with the highest probability of transmission tothose with the lowest, and/or from the channels with the lowestprobability of false detection to those with the highest. As soon asenergy is detected, receiver 38 adjusts to Mode 2 to further evaluatethe detected energy.

FIG. 6 is a flow diagram illustrating multi-tiered method 70 ofoperating receiver 38. This method is implemented in firmware and/orhardware of microprocessor 64 of receiver 38. In one embodiment of theinvention, multi-tiered method 70 includes Tier 1, Tier 2, and Tier 3,where each tier corresponds to a mode of operating receiver 38. Inaddition, method 70 enables control over the number of false detectionsthat occur, thereby reducing the energy consumed by the detection ofundesired RF signals, by raising channel thresholds 72 and 74,periodically reducing thresholds 76, skipping Tier 1 operation 77, andadjusting modulation sensitivity 78. After receiver 38 has sequentiallyevaluated the RF energy with Tiers 1, 2, and 3, receiver 38 opens acommunication session (step 80) with programmer 18. After thecommunication session is complete, receiver 38 ends the session (step82), and returns to Tier 1 operation.

Multi-tiered method 70 begins at Tier 1 with receiver 38 in Mode 1,which is a very low power receiver mode that utilizes only a portion ofreceiver 38, including a mixer, and an intermediate frequency (IF)channel of RF circuitry 54 and RSSI 56. In one embodiment, the bandwidthof receiver 38 is set to receive two consecutive channels plus 10 kHz.Here, receiver 38 samples the energy on each pair of channels over abrief period of time (such as 150 microseconds) and determines whether athreshold level of energy is present in that pair of channels during thesample period. This is done by passing any received RF energy to RSSI56, which then determines the strength of any received RF signal.Telemetry digital hardware 52 receives the strength indication from RSSI56 and stores each RSSI value in RAM corresponding to the appropriatechannel pairs. Telemetry digital hardware 52 also contains a thresholdvalue stored in RAM locations for that channel pair that defines theminimum RSSI value that will trigger Tier 2 for each channel. After eachpair of channels has been sampled, the RSSI values for the channel pairare compared to the corresponding threshold values by microprocessor 64.For any channel pair in which the RSSI value exceeds the thresholdvalue, the energy on that channel is further scrutinized by enteringTier 2. Tier 2 and Tier 3 processing are only performed after anydetected energy has passed Tier 1 scrutiny. In this way, significantenergy savings can be realized because higher energy consuming receivermodes are not activated unless a potential RF transmission is present ona channel.

To conserve power when in Tier 1, receiver 38 is operated so that thetotal receiver on-time per channel pair is very brief (such as 150microseconds). Because this receiver on-time is shorter than thelistening periods of RF transmission 40 (during discover period 44 andLBT period 46), it is possible that receiver 38 could miss transmission40 by turning on at a time when programmer 18 is not transmitting. Thepresent invention solves this problem by repeating the scan of all pairsof channels after a fixed period to ensure that one of the channelsamples will be taken during burst period 42 of RF transmission 40, iftransmission 40 is being broadcast. There are many available samplingperiods that can be selected that ensure that burst period 42 will bereceived, as illustrated in Table 1.

TABLE 1 Nth Frame Sampling Min. Sampling Max. Sampling Ave. Sampling N =Period (sec) Period (sec) Period (sec) 1 2.7 3 2.85 2 8.4 8.7 8.55 314.1 14.4 14.25 4 19.8 20.1 19.95 5 25.5 25.8 25.65 6 31.2 31.5 31.35 736.9 37.2 37.05 8 42.6 42.9 42.75 9 48.3 48.6 48.45 10 54 54.3 54.15 1159.7 60 59.85 12 65.4 65.7 65.55 13 71.1 71.4 71.25 14 76.8 77.1 76.95

Table 1 shows the first fourteen time periods during which a secondchannel sample could be taken to ensure that one of the two samples istaken during burst period 42, if present. Table 1 is based upon anembodiment in which burst period 42 is 3 seconds, discover period 44 is2 seconds, and LBT period 46 is 0.7 seconds. In the table, N representsthe number of frames (a frame is defined by one burst period 42, onediscover period 44, and one LBT period 46) after the first sample. Forexample, for N=1, the second sample would take place one frame after theframe of the first sample, and for N=3, the second sample would takeplace three frames after the frame of the first sample. For each frame,Table 1 identifies a minimum sampling period, a maximum sampling period,and an average sampling period for this embodiment. The minimum samplingperiod is the minimum time needed between the first and the secondsample to ensure that one of the samples will occur during burst period42, if present. Similarly, the maximum sampling period is the maximumtime needed between the first and the second sample to ensure that oneof the samples will occur during burst period 42, if present. Theaverage sampling period is the average of the minimum and the maximumsampling period, which will also ensure that one of the samples occursduring burst period 42, if present.

To further conserve energy consumption, the sampling periods may bevaried based upon factors such as the time of day, patient P activitylevel, or any other measurable or detectable criteria. For example, IMD12 may conserve energy during the night by increasing the time betweensamples, while increasing response time during the day by decreasing thetime between samples. For example, if the day sampling period were 8.55seconds, the night sampling period may be 19.95 seconds. Similarly, IMD12 can also use activity sensors or other measurable or detectablecriteria to dynamically adjust the sensing periods.

If any of the RSSI measurements obtained during the Tier 1 samplingperiods exceeds the corresponding channel threshold values, receiver 38enters Tier 2, as shown in FIG. 6. In Tier 2, receiver 38 is operated inMode 2, which also utilizes only a portion of receiver 38, such as amixer and an IF channel of RF circuitry 54, RSSI 56, and correlator 60,but takes a somewhat longer sample of the energy, thereby consuming moreenergy than Tier 1. In one embodiment, when operating in Mode 2, thebandwidth of receiver 38 is set to receive signals from a single channelplus 30 kHz. Any detected energy present on that channel is thenevaluated to determine whether its detected modulation corresponds to anexpected modulation of RF transmission 40.

Receiver 38 is centered on one of the two (or more) channels, forexample channel P5. Receiver 38 takes a sample of the RF energy presenton that channel for a period of time, such as 1.72 milliseconds. Becausereceiver 38 knows when burst period 42 was received in Tier 1, anappropriate period of time between that sample and the Tier 2 sample canbe calculated based upon the time periods shown in Table 1. If themagnitude of the energy on this channel does not exceed the thresholdvalue, then receiver 38 adjusts to the second channel and repeats theprocedure.

When operating at Tier 2, receiver 38 not only determines on whichchannel the potential RF transmission is, but also evaluates thetransmission for the appropriate modulation or pattern. To do so,receiver 38 provides the sample (for example 1.72 milliseconds) of thetransmission, if any, to converter/correlator 60. Correlator 60 comparesthe data pattern in the sample to the appropriate data pattern for burstperiod 42 (such as a repetitive 0-1 pattern, or any other desired bitpattern). If correlator 60 determines that the data matches theappropriate pattern, the received RF energy is likely to be RFtransmission 40 from programmer 18. As a result, receiver 38 initiatesTier 3 and powers up receiver Mode 3. If, however, correlator 60determines that the received data (if any) is not correlated, andtherefore is not RF transmission 40, receiver 38 returns to Tier 1 whereit continues to periodically perform a channel scan.

In an alternate embodiment, receiver 38 samples for a variable period oftime, rather than for a fixed period. Receiver 38, operating at Tier 2,begins evaluation of the data pattern contained in the potential RFtransmission. If receiver 38 determines that the pattern is notappropriate, it returns to tier 1 operation. However, if the appropriatepattern is initially received, receiver 38 continues to evaluate thepattern until either the pattern diverges from the appropriate patternor a predetermined period of time has passed. After such period of time,receiver 38 adjusts to receiver Mode 3 for further evaluation of thetransmission.

Many other criteria could also be used to evaluate RF energy in Tier 2of the present invention. For example, receiver 38 could detect AM or FMmodulation, or any other recognizable characteristic of an RFtransmission. In alternate embodiments, Tiers 1 and 2 may be combinedinto a single step, or one of Tiers 1 or 2 could be skipped entirely.

To maintain high sensitivity of receiver 38, it is desirable to allow acertain number of false signals to pass Tier 1 scrutiny. However, themore often receiver 38 is triggered to go beyond Tier 1, the more energyis consumed, which in turn reduces overall battery life. A balancebetween these conflicting goals is found through adaptive channelthresholds. Receiver 38 can periodically adjust the channel thresholdsto maintain them just above the noise level or other interferencepresent in each channel.

As previously described, receiver 38 stores threshold values in RAM foreach channel pair that identify the minimum strength a signal must have(as measured by RSSI 56) before it will trigger receiver 38 to move intoTier 2. At some point, noise present in the channel may exceed thethreshold value, which will result in a falsing that will trigger thetransition to Tier 2, where receiver 38 will analyze the noise signaland determine that the noise does not have the appropriate modulation.As a result, receiver 38 will raise the threshold value (step 72) to alevel greater than the noise signal received. However, to maintain thesensitivity of receiver 38, the channel threshold values areperiodically decreased (step 76). The period of time in which thethreshold values are decreased (step 76) depends on a desired falsingrate. A high falsing rate corresponds to greater battery consumption butincreased sensitivity. A low falsing rate corresponds to lower batteryconsumption but decreased sensitivity. In one embodiment, receiver 38has a 10% desired falsing rate for Tier 1 sensitivity, and receiver 38will gradually adjust the threshold values to achieve this desiredfalsing rate.

Receiver 38 may also increase the thresholds after detecting that afalse signal has caused receiver 38 to enter Tier 3 (step 74). In thisway, receiver 38 can avoid repeatedly detecting and analyzing a signalalready determined not to be RF transmission 40.

If noise causes receiver 38 to false at a rate higher than the desiredfalsing rate, FIG. 6 illustrates a provision for skipping Tier 1evaluation (step 77) altogether for a period of time. In this scenario,channel scans would be performed by Tier 2, which would evaluate energyon each channel to determine whether it exceeds the threshold value andwhether it is correlated with the appropriate modulation.

In an alternate embodiment, threshold values could be adjusted basedupon a time averaged power consumption. In this embodiment, receiver 38would keep track of an average power consumption over a period of time,such as hours, days, or months. Average power consumption could beestimated by receiver 38 by keeping track of the amount of time receiver38 is ON. In this way, higher falsing rates could be permitted for ashort period of time as long as the average power consumption wasmaintained within an acceptable amount. Furthermore, receiver 38 couldthen be controlled in such a way that it would not consume more than aspecified maximum percentage of the battery power of IMD 12.

Once a received signal has passed both Tier 1 and Tier 2 scrutiny, thatis, once the signal has the appropriate amplitude and modulation, it ishighly likely that the signal is RF transmission 40. As a result,receiver 38 enters Tier 3 in which receiver 38 now operates in Mode 3.Receiver 38 is adjusted so that it receives signals from only a singlechannel bandwidth. In this mode receiver 38 draws more current andsamples the energy for a longer period of time than in either of theprevious modes, thereby consuming more energy. Once full receiver 38 hasbeen powered up, it samples the received transmission to determine whatmessage is being sent. If receiver 38 determines that communication isnot appropriate, it raises the threshold value (step 74) for thatchannel and returns to Tier 1. Alternately, if receiver 38 determinesthat the transmission was not RF transmission 40, receiver 38 may alsoor alternatively adjust the modulation sensitivity of Tier 2 (step 78).For example, receiver 38 could adjust correlator 60 used in Tier 2 torequire that a received signal be more highly correlated before it willbe allowed to pass to Tier 3. After a period of time, receiver 38 wouldthen return the modulation sensitivity to the original settings toachieve optimum sensitivity. In the same way that the Tier 1 falsingrate can be adjusted, Tier 2 can also be adjusted to conform to thedesired modulation falsing rate, by adjusting the modulation sensitivityof Tier 2. In one embodiment, the Tier 2 modulation falsing rate is 5%,such that receiver 38 adjusts the modulation sensitivity up or downuntil 5% of all RF signals that trigger Tier 3 are determined to not beRF transmission 40.

Once Tier 3 evaluation is complete, such that receiver 38 determinesthat communication is appropriate, receiver 38 opens a communicationsession with programmer 18 (step 80). After the communication session iscomplete, receiver 38 ends the communication session (step 82). Receiver38 then resumes Tier 1 operation.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. Each tier of the multi-tiered method ofoperating receiver 38 could utilize a variety of detection schemes todetermine when an external device is attempting to initiate RFcommunication with it. For example, receiver 38 at Tier 1 could sampleor monitor for any type of signal, pattern, modulation, amplitude, orphase change. Furthermore, receiver 38 at Tier 1 could monitor forcommunications in other RF bands, or even non-RF communications such asan ultrasonic signal transmitted by a speaker in programmer 18 andreceived by a piezoelectric sensor coupled to receiver 38. Similarly,receiver 38 at Tiers 2 and 3 could also sample or monitor for any typeof signal, pattern, modulation, amplitude, or phase change. The presentinvention is also not limited to a three-tiered method, but rather couldbe implemented with any number of receiver modes. The present inventionhas been described with reference to implantable medical devices andexternal units. It is recognized that in some situations it would bedesirable to use the present invention for communication betweenimplantable medical devices, between external units, among a wirelessnetwork of implantable or external devices, or to reverse the roles ofthe implantable medical device and the external unit.

1. A method for receiving an unscheduled communication by operating areceiver of a medical device over a communication band having aplurality of frequency-divided communication channels, saidcommunication band divided into a plurality of bandwidths, eachindividual one of said plurality of bandwidths comprising a plurality ofindividual ones of said plurality of communication channels, comprisingthe steps of: operating said receiver in a first mode, comprisingdetecting radio frequency energy over a threshold sequentially over eachindividual one of said plurality of bandwidths; operating said receiverin a second mode if radio frequency energy over said threshold isdetected, comprising identifying, in an individual one of said pluralityof bandwidths in which said radio frequency energy over said thresholdhas been detected, an individual one of said plurality of communicationchannels having a radio frequency transmission; and opening acommunication session to receive said radio frequency transmission onsaid individual one of said plurality of communication channels.
 2. Themethod of claim 1, wherein the operating the receiver in a first modestep further comprises, evaluating a signal strength of the radiofrequency energy.
 3. The method of claim 2, wherein each individual oneof the plurality of bandwidths comprises a pair of individual ones ofsaid plurality of communication channels, and the operating the receiverin the first mode step further comprises: sampling the pair ofindividual ones of the plurality of communication channels of anindividual one of the plurality of bandwidths for radio frequencyenergy; determining a first strength value representative of a relativestrength of the radio frequency energy sampled in the sampling step;comparing the first strength value to a threshold value.
 4. The methodof claim 3, wherein the operating the receiver in the first mode stepfurther comprises: waiting for a period of time based upon a framelength of the communication; sampling the pair of individual ones of theplurality of communication channels of the individual one of theplurality of bandwidths for the radio frequency energy after the periodof time; determining a second strength value representative of a secondstrength of the radio frequency energy sampled in the sampling step;comparing the second strength value to the threshold value.
 5. Themethod of claim 3, wherein the operating the receiver in a second modestep further comprises the step of evaluating a modulation of one of thecommunication channels of the pair of individual ones of the pluralityof communication channels of the individual one of the plurality ofbandwidths.
 6. The method of claim 5, wherein the operating the receiverin a second mode step is only initiated if the first strength valueexceeds the threshold value in the operating the receiver in the secondmode step further comprises: sampling each individual one of the pairsof communication channels of the individual one of the plurality ofbandwidths for radio frequency energy; determining a second valuerepresentative of a strength of the radio frequency energy in eachindividual one of the pairs of communication channels; comparing thesecond value to the threshold value for each of the pairs ofcommunication channels; evaluating a modulation of radio frequencyenergy of any channel that exceeds the threshold value is correlated toa defined modulation.
 7. The method of claim 6 further comprisingincreasing the threshold value if the modulation is not correlated tothe defined modulation.
 8. The method of claim 7, further comprisingdecreasing the threshold value after a threshold decrease period.
 9. Themethod of claim 7, further comprising calculating the threshold decreaseperiod to achieve a first falsing rate corresponding to a number oftimes an individual one of the pair of communication channels of theindividual one of the plurality of bandwidths is erroneously determinedto contain a radio frequency transmission.
 10. The method of claim 5,further comprising the step of operating the receiver in a third mode,after the operating the receiver in a second mode step, comprisingevaluating data contained within individual ones of the pair ofcommunication channels of the individual one of the plurality ofbandwidths determined in the operating the receiver in the second modestep to contain a radio frequency transmission.
 11. The method of claim10, wherein the operating the receiver in the third mode step furthercomprises evaluating a message contained within the radio frequencytransmission to determine whether further communication is appropriate.12. The method of claim 11, further comprising increasing the thresholdvalue if further communication is not appropriate.
 13. The method ofclaim 11, further comprising increasing a selectivity of the evaluatingstep if further communication is not appropriate.
 14. The method ofclaim 13, further comprising decreasing the selectivity of theevaluating step after a modulation adjustment period.
 15. The method ofclaim 14, further comprising calculating the modulation adjustmentperiod to achieve a desired second falsing rate corresponding to anumber of times an individual one of the pair of communication channelsof the individual one of the plurality of bandwidths is erroneouslydetermined to contain a radio frequency transmission.
 16. A medicaldevice that conducts radio frequency communication over a communicationband comprising a plurality of frequency-divided communication channels,comprising: implantable medical device circuitry; a receiver operativelycoupled to the implantable medical device circuitry and operating in afirst mode and a second mode, the receiver detecting radio frequencyenergy over a first bandwidth when operating in the first mode anddetecting radio frequency energy over a plurality of second bandwidthswhen operating in the second mode, the first bandwidth being narrowerthan the communication band and broader than the individual ones of theplurality of second bandwidths, the first bandwidth comprising a firstsubset of the plurality of communication channels; wherein, with thereceiver operating in the first mode, the implantable medical devicecircuitry compares the radio frequency energy detected over the firstbandwidth against a threshold; wherein the first bandwidth is shifted toincorporate a second subset of individual ones of the plurality ofcommunication channels if the threshold exceeds the radio frequencyenergy, the second subset incorporating at least some differentindividual ones of the plurality of communication channels compared withthe first subset; wherein the receiver switches from the first mode tothe second mode if the radio frequency energy exceeds the threshold;wherein, after the receiver switches to the second mode, the implantablemedical device circuitry identifies an individual one of the pluralityof communication channels of the first bandwidth having a radiofrequency transmission; and wherein the receiver receives data via theradio frequency transmission on the individual one of the plurality ofcommunication channels of the first bandwidth having the radio frequencytransmission.
 17. The medical device of claim 16, wherein the firstbandwidth comprises a pair of said plurality of communication channels.18. The medical device of claim 17, the implantable medical devicecircuitry identifying, when the receiver is in the second mode, theindividual one of the plurality of communication channels bysequentially correlating a modulation of the radio frequency energy overindividual ones of the plurality of second bandwidths with apredetermined modulation.
 19. The medical device of claim 18, wherein,if the modulation correlates with the predetermined modulation, theimplantable medical device circuitry evaluates the radio frequencyenergy, and wherein, based on the evaluation, the receiver furtheroperates in a third mode, the receiver detecting radio frequency energyover a third bandwidth when operating in the third mode, the thirdbandwidth being narrower than the individual ones of the plurality ofsecond bandwidths and comprising one of the plurality of communicationchannels, the receiver receiving the data via the radio frequencytransmission when operating in the third mode.
 20. A medical devicesystem for receiving data transmissions over a plurality of transmissionchannels, comprising: means for determining whether RF energy withinpairs of transmission channels of the plurality of transmission channelsexceed a first energy threshold; means for determining whether RF energywithin each individual transmission channel of pairs of transmissionchannels determined to exceed the first energy threshold exceed a secondenergy threshold; means for determining whether a data pattern of a datatransmission associated with an individual channel determined to exceedthe second energy threshold matches a data pattern threshold and foridentifying the individual transmission channel as containing a desireddata transmission in response to the data pattern matching the datapattern threshold; means for verifying the identifying of the individualtransmission channel as containing a desired data transmission; andmeans for opening a communication session to receive the desired datatransmission on the individual transmission channel.
 21. The system ofclaim 20, further comprising means for determining probabilities oftransmission for each of the plurality of transmission channels and foradjusting an order of the determining, by the means for determiningwhether RF energy within pairs of transmission channels exceed the firstenergy threshold, in response to the determined probabilities.
 22. Thesystem of claim 20, further comprising means for determining, for eachof the transmission channels, a probability of not being verified by themeans for verifying when the transmission channels are identified ascontaining a desired data transmission, and for adjusting an order ofthe determining, by the means for determining whether RF energy withinpairs of transmission channels exceed the first energy threshold, inresponse to the determined probabilities.